1. Field of the Invention
The present invention relates generally to optical waveguide assemblies, and in particular relates to such assemblies that use large-mode-area optical fibers having reduced stimulated Brillouin scattering.
2. Technical Background
Optical waveguides support one or more optical guided modes and are used in a variety of optical systems and applications. Optical fibers are one type of optical waveguide and are used to carry large amounts of information over long distances in telecommunication systems. Optical fibers are also use to carry large amounts of optical power in fiber amplifiers and fiber laser systems.
Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty in many optical-fiber-based transmission systems. SBS occurs when an input lightwave traveling through the fiber generates an acoustic wave through the process of electrostriction. The acoustic wave causes a periodic modulation of the fiber refractive index, which operates as a Bragg grating from which photons can be scattered. The result is selective amplification of a slightly frequency-downshifted lightwave (Stokes wave) that propagates in the direction opposite of the input lightwave.
In many fiber-based optical systems, launching large amounts of optical power into the fiber while maintaining a high signal-to-noise ratio (SNR) is desirable. However, as the launch power or signal power of the incident signal increases, it may exceed the SBS threshold power and cause part of the signal power to reflect back to the transmitter due to SBS. In addition, the scattering process increases the noise level at the signal wavelength. The combination of decrease in signal power and increase in the noise lowers the SNR and leads to performance degradation of the optical system. To first order, SBS is an increasing function of 1/λ so that its effects are particularly problematic for relatively short-wavelength (and in particular, ultraviolet (UV)) applications.
There are a number of ways to mitigate SBS effects. One approach involves changing the concentration of fluorine dopant along the fiber length, which is not always possible or desirable. Another approach involves providing the fiber with an axially varying strain profile. Another approach involves increasing the source bandwidth. Yet another approach is to vary the temperature along the length of the fiber. These approaches are largely independent of system size.
Besides SBS, there are other obstacles to obtaining good optical performance from high-power fiber-based systems, such as the availability of sufficient pump power, and thermal management. With respect to the latter, optical fibers are not 100% efficient and absorb some of the input light. This light is converted to heat, which must be removed from the system to prevent damaging the system or to prevent performance degradation. While these obstacles can generally be overcome by increasing the system size, they are problematic if the system needs to be compact. Unfortunately, most high-power waveguide-based systems need to be compact so that they can readily fit into relatively small enclosures when deployed in the field.
Many high-power fiber-based systems employ large mode area (LMA) optical fibers that normally support multiple modes but are made to operate in single mode. Single-mode operation provides for a diffraction-limited output beam useful for a number of important applications, such as fiber amplifiers and fiber lasers. Because an LMA optical fiber typically supports multiple modes, single-mode operation is achieved by “mode stripping,” wherein all modes, save for the fundamental mode, are forced out of the fiber. This is typically accomplished by subjecting the fiber to bending at a given radius. However, fiber bending at small radii can mechanically damage the fiber, can cause the light from the stripped modes to damage the optical fiber, and can also alter the strain profile and adversely influence the mitigating effect on SBS.